Ring laser gyros are devices designed to accurately detect changes in direction of the gyro frame. Their operation involves detecting apparent path length changes for light beams counterrotating around the ring. It is extremely critical that the coefficient of thermal expansion of a material intended for use as the frame be as small as possible. Hence, although some compensation can be made for thermal expansion in the design of active mirrors positioned on the frame, it is generally preferred that the coefficient of thermal expansion over the temperature range of -50.degree. to +100.degree. C. be held less than 0.66.times.10.sup.-7 /.degree.C.
In addition to requiring a low coefficient of thermal expansion, the material must also display optical transparency. Clarity is demanded not only for accurate measurement of the frame, but is most critical when the material is used for the recombination prism. The function of this prism is to combine the light beams into a single unit which produces a fringe pattern on a detector when the frame is rotating. Light scatter due to haze and/or refractive index in homogeneity must be kept at a very low level to prevent spurious signals in the detector.
Finally, long term thermal stability and resistance to permanent deformation during thermal cycling are also necessary for the material to be used for the frame. The latter characteristic is especially important since poor resistance to deformation can impose essentially impossible requirements on the structure of the frame. Thus, permanent deformations resulting from thermal cycling will necessitate repeated recalibration of the path length of the gyro.
The basis for the production of glass-ceramic articles can be found in U.S. Pat. No. 2,920,971. As is explained therein, glass-ceramic or semicrystalline ceramic bodies, as such have been variously termed, are formed through general steps: (1) a glass-forming batch, in which a nucleating agent is normally included, is melted; (2) that melt is cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry simultaneously shaped therefrom; and (3) that glass body is subjected to a predetermined heat treatment to cause the in situ generation of crystals. Very frequently the crystallization is divided into two stages. In the first step the precursor glass body is heated to a temperature within or slightly above the transformation range for a period of time sufficient to develop nuclei in the glass. Thereafter, the nucleated glass is heated to a higher temperature, often approaching and exceeding the softening point of the glass, to cause the growth of crystals on the nuclei. This two-step method typically produces glass-ceramics of higher crystallinity wherein the crystals are more uniformly sized. (The transformation range has generally been defined as that temperature at which a melt is converted into an amorphous solid, that temperature normally being deemed to lie in the vicinity of the annealing point of a glass.)